Transparent optical networks are in the interest of network operators for many years. They allow for direct optical switching of optical channels through a meshed network. For the operation of such optical networks monitoring is important, in order to identify the degradations incurred by signals which are being transmitted across the networks. There are various sources of degradation which accumulate during propagation over the optical transmission path and these performance limiting effects are to be identified and quantified. Different monitoring techniques have been proposed.
Furthermore, the level of degradations incurred by signals in an optical transmission network may vary in time. Such variations may be due to changes in the environmental conditions of the network, e.g. temperature, surrounding electrical fields, etc. In addition, the level of degradations may depend on the actual load in the network, i.e. on the actual amount of traffic carried within the network.
Linear distortions within an optical channel of an optical network may be compensated by means of digital signal processing. From the equalizer settings at an optical receiver, it is possible to extract information about chromatic dispersion (CD) and polarization mode dispersion (PMD) of the respective optical channel. This information may be used to compensate the linear distortions of an optical channel. These methods for monitoring and compensating linear distortions are particularly relevant when using coherent optical receivers. Furthermore, it should be noted that the use of orthogonal frequency division multiplexing (OFDM) allows for efficient equalization in the frequency domain.
Apart from linear distortions, critical parameters of the optical channel, which limit the achievable throughput of the channel, are the power of amplified spontaneous emission (ASE) noise and further distortions due to non-linear effects. The signal degradation which results from these effects accumulates during propagation along the optical transmission line and can typically only be fully characterized at the final destination, i.e. at the optical receiver.